Optical waveguides provide a maximum velocity of propagation for light occupying a relatively narrow range of wavelengths or optical frequencies. The point at which this maximum velocity is achieved depends on the design and material composition of the waveguide. For wavelengths outside this range, the velocity of propagation falls slightly as the wavelength moves further away from the wavelength of maximum velocity. This lower velocity manifests itself as a variable delay through a long optical waveguide path, where the delay depends on the light frequency or wavelength.
The derivative of the propagation delay with respect to the wavelength or frequency of light propagated through an optical waveguide is referred to as chromatic dispersion. An optical signal is said to have positive chromatic dispersion polarity if the higher-frequency components are delayed with respect to lower-frequency components, while negative chromatic dispersion polarity refers to the opposite situation. It is to be noted that other types of dispersion exist, such as polarization mode dispersion for which the two polarizations (horizontal and vertical) of light are propagated at two different velocities in an optical waveguide. In the following, the term “dispersion” encompasses either or both types of dispersion.
In an optical telecommunications system, dispersion may be problematic. In particular, an optical carrier that is modulated with data contains information-related sideband optical frequencies differing slightly from the carrier's optical frequency. Specifically, two sidebands (an upper one and a lower one) are present on either side of each carrier optical frequency since the data typically amplitude modulates the optical carrier. If the carrier frequency is not within the range of frequencies conducive to maximal propagation velocity, the delay experienced by the upper sideband will differ from the delay experienced by the lower sideband. Therefore, the upper and lower sidebands will be phase shifted by an amount determined by the differential delay between the upper and lower sidebands, which increases with sideband optical frequency. At some sideband frequencies, this phase shift can interfere destructively so as to reduce the amplitude of these sideband optical frequencies at the receiving end. As the delay between optical frequency components in the two sidebands approaches 180°, total cancellation will occur. Those skilled in the art will appreciate that this is an undesirable scenario, which may lead to information loss if left uncorrected.
Clearly, therefore, it would be advantageous to correct dispersion in an optical transmission system by ensuring that the system, taken as a whole, operates over an optical path with a negligible propagation velocity differential at optical frequencies around the carrier optical frequency of the signal being propagated. This would allow the upper and lower sidebands to experience the same propagation delay, thereby avoiding the destructive interference described above.
An example of a conventional dispersion compensation system is a length of “compensating fiber” which is inserted into a transmission path. The length and properties of the compensating fiber are chosen so that the latter will have a desired compensatory effect on a dispersion-affected signal propagated therethrough. However, the optical frequency range (or “window”) for which the dispersion remains within a given tolerance is relatively narrow. This is because the optical frequency of maximum propagation velocity through the compensating fiber is different from the optical frequency of maximum propagation velocity through the fiber being compensated and hence the rate of change of dispersion with optical frequency of the compensating fiber does not cancel the rate of change of dispersion with optical frequency of the fiber being compensated, except within a very narrow range of frequencies. Although the dispersion of the compensating fiber may be equal but opposite at a given optical frequency to the dispersion present in a dispersion-affected signal propagated, the slope of the dispersion (d(dispersion)/d(optical frequency)) is of the same polarity for all dispersive structures with a propagation velocity that passes through a maximum rate of change at a particular optical frequency.
In order to correct the dispersion at multiple frequencies, it has been proposed to use a concatenated mixture of fiber types, each targetting a specific window of optical frequencies. However, in order for this solution to work as intended, the degree of dispersion affecting each window of optical frequencies must be known ahead of time. In a photonically switched network, this requirement is impossible to satisfy, since, at different times, adjacent wavelengths in a WDM stream may have different ancestries and suffer from different impairments. Therefore, a conventional approach cannot be used to correct dispersion in photonically switched networks, unless rigorous compensation is carried out on every span between switches, and even then the residual errors in compensation will build up span-by-span as the signals propagate through the network.
With the aim of providing adequate dispersion compensation for a photonically switched multi-channel optical signal, above-referenced U.S. patent application Ser. No. 09/965,810 describes a dispersion discrimination and compensation system including a dispersion discrimination subsystem shown in FIG. 1. Suitable implementations of a dispersion discrimination subsystem 12 are described in above-referenced U.S. patent application Ser. No. 09/842,236. For example, a dispersion discrimination subsystem 12 may include two arms 20, 22, one of which adds positive dispersion and the other of which adds negative dispersion. Each arm is fed a portion of the live traffic or other optical signal from the dispersive transmission path under test that is tapped by a splitter 18 connected to an optical fiber 14. The signals in each of the two arms 20, 22 enter a processing unit 24, where they are processed and compared to one another, resulting in the issuance of control signals 25 which are used to control, in this specific case, the amount of dispersion applied by a dispersion compensation subsystem (not shown in FIG. 1).
The design of the arms 20, 22 in the dispersion discrimination subsystem 12 is such that each arm introduces dispersion of an equal magnitude but of an opposite polarity for the one case where the signal drawn from the optical fiber 14 is devoid of dispersion. This will result in a balanced roll-off of spectral energy at high modulation frequencies in the detectors in both arms 20, 22, when fed from a dispersionless source or test signal. In the case where the signal drawn from the optical fiber 14 suffers from dispersion, the discriminator arm with like dispersion polarity will exhibit a lower roll-off frequency since its dispersion will add to the dispersion of the signal exiting the optical fiber 14, while the other arm will show a less severe roll-off due to some usually incomplete level of dispersion compensation. This can be detected as a differential spectral energy density when a scanning filter is scanned in receivers, comprised in the processing unit 24, attached to the two arms 20, 22, and the results compared.
Thus, it is seen that the processing unit 24 can determine the polarity of the dispersion of the signal (prior to entering the arms 20, 22) by noting which arm has the lower roll-off frequency or sideband spectral density. In addition, U.S. patent application Ser. No. 09/842,236 describes how the processing unit 24 can compute not only the polarity, but also the magnitude of the dispersion present in the signal drawn from the optical fiber 14 by determining the sideband optical frequency at which the first unbalance beyond a set threshold occurs.
From the above, it is apparent that detection of an identical roll-off in both arms 20, 22 is indicative of the two arms having had an equal but opposite effect, from which it is inferred that the signal drawn from the optical fiber 14 must have been afflicted with no discernible amount of dispersion. It is this condition that is sought by the feedback loop involving the dispersion discrimination subsystem 12 and the dispersion compensation system.
The above described system functions adequately when the signal to be discriminated (the “channel under test”) has a narrow range of possible optical frequencies. However, when the channel under test has a broad range of possible optical frequencies, the conclusion that an equal reading from both arms 20, 22 implies a dispersion-free signal is not always true. Specifically, this conclusion is only valid when the channel under test is centered about a nominal operating frequency, this nominal operating frequency being dependent on the particular length and construction of the arms 20, 22. Practically speaking, and depending upon the precision required, there may be a range of only a few ITU grid slots (of 50, 100, 200 GHz) of possible center optical frequencies for which an equal reading from both arms 20, 22 of the dispersion discrimination subsystem 12 truly does indicate that the channel under test is free of dispersion. For channels centered about higher or lower optical frequencies outside this range, the dispersion discrimination subsystem 12 will not produce an equal reading when the channel under test has zero dispersion.
The reason for this is that the arms 20, 22 are fabricated from different fibers in order to allow dispersion of a different polarity but same magnitude to be added to the channel under test. Each of the arms 20, 22 of the dispersion discrimination subsystem 12 therefore exhibits a particular optical frequency for maximum propagation velocity; however, the second differential of the delay (or the differential of the dispersion with optical frequency) is positive for both arms 20, 22. Hence, the dispersion discrimination subsystem 12 can only be “balanced” at or close to one optical frequency, with the arms 20, 22 showing differing magnitudes of dispersion at other optical frequencies. Away from the optical frequency at which the dispersion discrimination subsystem 12 is balanced, the processing unit 24 will indicate that it is balanced when in fact the dispersion of the channel under test is equal to the (non-zero) difference between the dispersion applied by the two arms 20, 22 at that frequency.
This operational characteristic of the dispersion discrimination subsystem 12 may be better understood upon consideration of an example. Specifically, FIG. 2A shows a “response characteristic” of the output of each of the two arms 20, 22 as a function of dispersion of the channel under test, when the channel is centered at the nominal operating frequency of the dispersion discrimination subsystem 12 (in this case at around 214 THz, where 1 THz=1012 Hz=1000 GHz) and when a particular length and characteristic of dispersion compensating fiber is used in each arm. An example of a suitable “response characteristic” is the −3 dB frequency (low-frequency roll-off) of the received signal spectral density relative to the known transmitted spectral content. The reader is referred to above-referenced U.S. patent application Ser. No. 09/842,236 for other suitable response characteristics.
It will be appreciated that the level of dispersion at which the peak of the response characteristic of a given arm occurs is representative of the opposite of the dispersion applied by that particular arm. Thus, from FIG. 2A showing the results for a discriminator optimized for operation at 214 THz, it is seen that, in this example, the positive dispersion arm 20 applies a dispersion of 0.6 ps/GHz and the negative dispersion arm 22 applies a dispersion of −0.6 ps/GHz. At very close to zero dispersion in the channel under test, the response characteristic of either arm has dropped off considerably, and, moreover, the response characteristics at the output of the two arms 20, 22 have the same value, meaning that the dispersion discrimination subsystem 12 is “balanced”. In other words, achieving an identical value for the recovered sideband optical frequency response characteristic in both arms of the dispersion discrimination subsystem 12 results from the two arms 20, 22 applying an equal but opposite overall effect, causing an overlap of their characteristics if the channel under test is free of dispersion upon being drawn from the optical fiber 14.
Another way of understanding FIG. 2A is as follows. As the transmission path dispersion departs from zero, it partially compensates one of the two arms 20, 22 of the dispersion discrimination subsystem 12, while adding dispersion to the other one of the two arms 20, 22, moving the cut off frequency higher for the arm it is partially compensating and reducing the cut-off frequency for the other arm. As the amount of dispersion on the transmission path increases, it eventually reaches a point where it is equal in magnitude but opposite in sign to that of the arm that it was compensating or alternatively that was compensating it and the cut-off frequency increases towards infinity. For dispersion beyond this magnitude, the arm that is of the opposite polarity to the dispersion being measured is now applying compensation to the sample from the transmission path but only partially compensates for the dispersion on the transmission path. In this case, the cut-off frequency falls but always remains above the cut-off frequency of the other arm, which is adding to the transmission path dispersion. By comparing the values of cut-off frequency (in this case −3 dB cut-off) of the two arms 20, 22 and knowing the level of dispersion added in the arms, it is possible to compute the level of dispersion and polarity of dispersion on the transmission path from which the channel under test is drawn.
However, if the channel under test is now centered about a considerably lower or higher frequency than the balance optical frequency for the same lengths and fiber types as above in the two arms 20, 22, the curve of the response characteristic versus the dispersion will be similar in shape to that shown in FIG. 2B. This figure shows different response characteristics over a range of 208 to 222 THz for the channel under test, where the nominal operating frequency of the dispersion discrimination subsystem 12 remains at 214 THz. This series of offset curves shows what happens as a result of the negative arm becoming less negative at the same time as the positive arm becoming more positive as the optical frequency increases, creating a fundamental unbalance in the action of the two arms 20, 22. More specifically, the two arms 20, 22 do not apply compensations that are of opposite polarities and equal magnitude.
More specifically, in the case of a 222 THz optical carrier, for example, it is seen that the positive dispersion arm 20 applies a dispersion of approximately 0.2 ps/GHz, while the negative dispersion arm 22 applies a dispersion of −1.1 ps/GHz. For zero dispersion in the channel under test arriving at the discriminator arms, the response characteristic of either arm has again dropped off considerably. However, the response characteristics of the two arms 20, 22 do not have the same value. This means that the dispersion discrimination subsystem 12 is not balanced for a channel under test centered at about 222 THz; rather, it can be seen that the response characteristics cross over when the channel under test has a dispersion of approximately −0.4 ps/GHz. In other words, the discriminator will indicate a zero dispersion measurement when in fact the channel under test has a dispersion of −0.4 ps/GHz.
Thus, it is seen that the pursuit of an identical response characteristic in both arms 20, 22 will cause a residual dispersion of −0.4 ps/GHz to be retained in the channel under test when the latter is centered at 222 THz. This situation is equally problematic in the case where the channel under test is centered about a considerably lower frequency than the center frequency of the channel for which the dispersion discrimination subsystem 12 is balanced (in this case, 214 THz). It should therefore be appreciated by those of ordinary skill in the art that the above described dispersion discrimination subsystem 12 provides biased results for channels centered about optical frequencies other than those falling within the narrow range where the discriminator is “balanced”. This, in turn, causes the application of erroneous levels of compensation by the dispersion compensation subsystem.
Since it is expected that the individual channels of dense wavelength division multiplexed (DWDM) optical signals will occupy increasingly wider ranges of the optical spectrum, it becomes apparent that available techniques for dispersion discrimination are no longer effective unless there can be provided some form of operational dependency on the center frequency of the channel under test.